Remove dodgy code that detects mp3 with as little as
a valid frame sync at the beginning. This was only used
in some unit tests in -good where there were only a few
bytes after the id3 tag. We now require at least two
frame headers.
Fixes mis-dection of text files with UTF-16 LE BOM as mp3.
https://bugzilla.gnome.org/show_bug.cgi?id=681368

We have to hold the streams-lock when iterating over all pads,
also the stream-lock of the pad is already locked when we receive
EOS.
Call gst_pad_event_default() for the correct default handling of
events.